I have never used Twitter but here is my understanding. Hopefully a user will correct me if I'm wrong. Twitter in my opinion is more of following the daily minutia of people, e.g., I'm attending a seminar, I'm eating a cheeseburger, I'm driving down the freeway, I just bumped my head......etc... The communication is one-way.
Many times people will Twitter with celebrities, people of renown etc., just to follow their activity. They will also follow the tweets of their friends. Facebook members (of which I am a member) generally are in communication with and following the activity of friends. Communication and friends are the key words here since many times people twitter the activity of people they have no personal relationship with nor do they communicate with them on Twitter.
I also think Twitter is more of an ego thing for many users. As a Twitter member you will know how many people are following you and some people like to attract as many followers as possible. The same occurs in Facebook for some people but not to the degree as in Twitter.